Derivative Instruments (Details 1) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Regulatory Assets [Line Items]
Regulatory assets related to derivatives	$ 1,271.2us-gaap_RegulatoryAssets	$ 1,108.5us-gaap_RegulatoryAssets
Derivative [Member]
Regulatory Assets [Line Items]
Regulatory assets related to derivatives	$ 14.7	$ 0.3

The amount for the individual regulatory asset as itemized in a table of regulatory assets as of the end of the period.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 980

-SubTopic 340

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382

Name:	us-gaap_RegulatoryAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant